As resinous intake parts for internal and external combustion engines of an automobile or the like, various ducts have been manufactured by blow molding and an intake manifold has been manufactured by a fusibility core method or welding of a plurality of parts. However, they have not been resinified as an air intake duct for the MSC but have been manufactured by aluminum die casting using a sandmold.
If the sandmold is used, however, a product becomes very rough with a maximum surface roughness of Rz.gtoreq.50.mu.m and a mean surface roughness of Ra.apprxeq.8 .mu.m. For this reason, it is necessary to perform a grinding work in order to keep a profile irregularity of a supercharger junction. Furthermore, a difference in level is easily caused by mismatch. Therefore, it is also necessary to perform a finishing work for a hose junction.
The reasons why a resin has not been used for the air intake duct for the MSC are as follows.
(1) It is hard to integrally mold a flange portion by the blow molding. PA1 (2) Few resin materials for the blow molding have performance, for example, strength, heat resistance and the like necessary for the air intake duct for the MSC. The resin materials having the performance such as strength, heat resistance and the like are expensive. Moreover, the surface roughness of an internal wall is deteriorated more than in existing items (manufactured by the aluminum die casting). PA1 (3) In the fusibility core method or the welding method of a plurality of parts, the investment for a mold and initial plant should be made greatly. In consideration of plant repayment, a cost is increased more than in the existing items.
An injection molding method using a fused resin and a fluid such as an inert gas or the like together (a gas assist injection method which will be hereinafter referred to as "GAI") has recently been applied variously as a method for molding a hollow bent tube at a low cost.
The merits of the GAI are as follows: "(1) A mold can be manufactured by simply adding comparatively inexpensive equipment to an ordinary mold for injection molding; (2) Since a flange portion can be molded integrally and a mean surface roughness is small, a grinding work is not necessary; (3) It is possible to select inexpensive resin materials suitable for the present product from a great variety of injection molding resins; (4) It is possible to obtain appearance and dimensional precision equivalent to or more than those of an ordinary injection molded product; (5) A low pressure molding method is used so that the life of the mold can be prolonged; and the like."
However, the mere GAI has disadvantages, for example, "(1) If a reinforcement (glass fiber or the like) is blended to such an extent that necessary performance is revealed, an internal wall becomes rough in the same manner as in the blow molding; (2) It is hard to obtain the same hollow rate as in existing items (manufactured by the aluminum die casting); and the like."
In consideration of these problems of the prior art, it is an object of the present invention to provide a hollow tubular body for an air intake duct which can be manufactured at a low cost and has an excellent internal smoothness. It is another object of the present invention to provide a mold capable of stably molding a hollow tubular body for an air intake duct having various sizes and shapes, and a method for molding the hollow tubular body.